The most common internal combustion engine in use today is the four-cycle type, either spark ignited or diesel, wherein there is one power stroke for every two revolutions of the crankshaft. This type of engine has a suction stroke by which it can aspirate its own air, and thus it is able to start and run without the aid of an external or auxiliary air supply. In contrast, the two-cycle internal combustion engine does not have a separate suction stroke since there is a power stroke for every revolution of the crankshaft. Consequently, the two-cycle engine must employ other means of exhausting and charging the cylinder with fresh air after each power stroke has been completed.
Typically, two-cycle engines use either an engine-driven compressor or crankcase air compression to force air into the intake ports of the cylinders. Crankcase compression is undesirable due to the tendency to mix lubricant oil present in the crankcase with the air which can cause excessive lubricant oil consumption and increase the amount of pollutants in the exhaust gases. Thus, it is more desirable to use an air compressor driven separately than from the engine to provide the necessary air supply for starting and running the engine.
When engine-driven compressors are used, the fuel consumption of the engine is increased by virtue of the horsepower absorbed by the compressor. Adding an exhaust gas-driven turbocharger in series with the engine-driven compressor is a substantial improvement. However, there is still a residual power drain on the engine caused by the engine-driven compressor and its gear train. Clutches between the engine-driven compressor and the engine have been used to disconnect the compressor from the engine when the turbocharger is capable of scavenging and charging the cylinders by itself. However, the addition of a clutch in the compressor drive train adds a very significant amount of cost and complication to the mechanical configuration of the engine.